The conventional rudder is positioned in the slipstream of a propeller and hence by creating an additional resistant component. When a rudder is not disposed behind a propeller but to remain in the same lateral plane with the propeller, it is left to arrange the rudder aside or in front of the propeller. In view of interference with a propulsion shaft, configuration of two or more rudders must be taken. On the other hand, Non-Patent Literatures 2 and 3 pay an attention to the stopping ability, and propose the adoption of a single-shaft propulsion with twin rudders for ship handling. It is stated therein that, upon request of a sudden stop at emergency, the two rudders cooperate at a right angle to a hull by taking position behind the propeller to block its slipstream, and to provide the vessel with a powerful stopping ability. This way of steering and stopping technique is not much different from the prior art, in a point that the rudder still acts as a resistance component in the slip stream of a propeller. As the prior invention of twin rudder, there is the invention disclosed in Patent Literature 1. The same invention prioritizes improvement in propulsive performance due to such a rudder plate that “two rudder plates are arranged in front of or aside the propeller, and does not concentrate on the stopping ability. On the other hand, configuration having two steering shafts is also disclosed in FIG. 12 of Patent Literature 1, and since a rudder plate rotates around a steering shaft included in a rudder plate face, the rudder plate cannot take position behind the propeller slipstream and hence a problem arises in the steering ability, particularly, at a slow vessel speed. This is problematic for domestic vessels and patrol boats which cannot receive assistance of tugboats. When the number of rudders is 2, utilization of a camber comes in sight, but Patent Literature 2 is limited to use the effect of camber in a twin rudder arrangement in propeller slipstream. At a rudder angle of 90 degree, it becomes necessary to also contrive a steering shaft driving mechanism, and Patent Literature 3 proposes an oil hydraulic driving mechanism which enables a rudder angle near 180 degree, using a rotary vane. Patent Literature 4 describes the proposal that the effect of straightening a propeller slipstream in a region sandwiched by two rudders is exerted, and a high propulsive efficiency can be realized. However, in this latter arrangement, since the rudders are arranged at the slipstream of a propeller, it seems that there is a limitation in improvement of the propulsive performance. Inter alia, in domestic vessels, since the support by tugboats inside the bay will not be expected, the turning ability at the slow ship speeds should be maintained by own ship handling. In the case of the rudder arranged outside of the propeller slip stream aiming to only higher propulsive efficiency, the special attention should be paid to the movement of the rudder during the steering motion and also mechanism and a steering method are the same. An invention which recognizes or suggests a solution by discriminating at the time of slow speed navigation and at the time of cruising concerning steering in this case has not been found out. In this respect, as a method for steering two rudders, FIG. 4 of Patent Literature 5 presents a “method for displaying a moving direction for a system of two rudders”. In this presentation the rudder position and a moving direction of a ship are displayed in ships having two rudders as such rudder arrangement of steering modes of (b) indicates forward right turning, and (e) right turning on the spot. However the present invention is not suggested by a positional relationship between a turning central position of two rudders and a propeller in propeller slipstream arrangement. In addition, there is proposed a ship in which two rudders are arranged on both sides of a propeller, for the purpose of shortening the length of the propeller and that of a stern rudder for expansion of space for a stein (Patent Literature 4). However, according to configuration shown in FIG. 8 of Patent Literature 4, it seems that there is limitation in a steering range, and it is difficult to create a deflected stream of a propeller slipstream.